I. Field of the Invention
The present invention relates to a spindle motor and a method of manufacturing the same, particularly to spindle motor for rotatably driving recording discs such as magnetic discs and optical discs and a method of manufacturing the same.
II. Description of the Related Art
Spindle motors have been known as motors to rotatably drive recording discs such as magnetic discs and optical discs in hard disc drives.
The conventional spindle motor mainly comprises a base plate, a stator provided on the base plate, a rotor to which the recording disc is attached, and a hydrodynamic bearing provided on the base plate for rotatably supporting the rotor hub relative to the base plate. The rotor includes a rotor hub to which the recording disc is attached, an annular yoke provided on the rotor hub, and an annular rotor magnet provided on the yoke opposite an outer circumference of the stator in the radial direction (for example, refer to Unexamined Patent Publications 2004-263863, H11-312359, and H06-178490).
The recording wave length and the track pitch of the recording disc have become remarkably smaller because the recording density has been improved to a large extent. Accordingly, deformation of the recording discs such as curling and deflection or vibrations of the rotating recording disc make it impossible to follow the head to desired tracks when recording or reproducing. Especially, since the head of the magnetic disc drive floats with a micro-amount of the float on the order of 10 nm, the head and the recording disc might come into contact with each other due to the deformation or the vibration of the recording disc. Therefore, in the spindle motor, it is very important to reduce the deformation of the rotor hub during the manufacturing process or when the recording disc is clamped to ensure the flatness of the recording disc. It is also very important to reduce the vibration of the rotor.
However, in some cases, the rotor hub is deformed during the process of assembling the rotor hub and the yoke. In this case, the rotor hub in Unexamined Patent Publication H06-178490 will be explained as an example. FIG. 11 is a view of a yoke 533 and a hub 531 at the conventional assembling. The rotor hub 531 includes a disc support cylindrical portion 535 for supporting the recording disc in the radial direction, an annular disc mount portion 536 formed on the outer circumference of the disc support cylindrical portion 535 to mount the recording disc thereon, and an annular projection 537 formed on a side of the disc mount portion 536 in the axially opposite direction to the recording disc and having an outer circumference to which the yoke 533 is fixed.
When the yoke 533 is assembled to the rotor hub 531, for example, as shown in FIG. 11(a), the rotor hub 531 is set into a jig 590 such that a disc receiving face 536b of the disc mount portion 536 comes into contact with the jig 590. Then, the yoke 533 is press-fitted around the outer circumference of the annular projection 537 of the rotor hub 531. An annular recess 538 is formed radially inward of the disc receiving face 536b to prevent the contact between the disc mount portion 536 and the recording disc, so that the strength of the disc mount portion 536 is deteriorated at this portion. Accordingly, the disc mount portion 536 may be deformed when the yoke 533 is press-fitted, depending on the positional relationship between the annular projection 537 and the disc receiving face 536b. Furthermore, since a load is applied to the disc receiving face 536b when the yoke 533 is press-fitted, the disc receiving face 536b might be flawed and the disc receiving face 536b might be deformed. Alternatively, as shown in FIG. 11(b), the outer circumference of the end of a disc support cylindrical portion 535 is supported by a jig 595 in order to protect the disc receiving face 536b. In this case, since the disc receiving face 536b is not supported when the yoke 533 is press-fitted, a bending moment is applied to the disc mount portion 536 so that the disc mount portion 536 is deformed.
As described above, if the disc mount portion is deformed during the manufacturing process, it cannot ensure the flatness of the disc receiving face, i.e., the flatness of the recording disc. In addition, in order to ensure the flatness of the recording disc, it is typically necessary to perform highly precise additional processing on the disc receiving face after the yoke is press-fitted, so that the cost of manufacturing becomes high. However, even if the highly precise additional processing is performed, residual stress at the deformation might remain. Consequently, stress relaxation of the rotor hub 531 occurs as deterioration with age so that micro-deformation is generated, thereby to progress curling or deformation of the disc. Therefore, when the yoke is press-fitted around the annular projection, it is necessary to prevent the deformation of the disc mount portion.
In contrast, if the yoke is fixed by adhesive, not by press-fitting, since a large load is not applied to the annular projection and the disc mount portion, it is possible to prevent the deformation of the disc mount portion. However, if the yoke is fixed by the adhesive, it is necessary to provide an applicator for the adhesive. In addition, in order to prevent the out gas from being generated from the adhesive, it is necessary to bake bonded products for a long time in a adhesive clean oven installed in the clean room after the hardening. In other words, it is necessary to provide the clean room and the clean oven as well as the coater if the adhesive is used. If production amounts of the spindle motors reach a level of several tens of thousands a day, it is necessary to consume great energy to operate the manufacturing apparatuses, so that the cost of manufacturing becomes high. From a standpoint of the Earth's environment protection, it is also necessary to reduce the power consumption during the manufacturing process. Therefore, during the manufacturing process of the spindle motor, it is preferable not to use the adhesive if possible. It is preferable to press-fit the yoke around the rotor hub.
Consequently, it is very important to prevent the deformation of the disc mount portion of the rotor hub when the yoke is press-fitted, from standpoints such as the performance of the spindle motor, the cost of manufacturing, and the Earth's environment protection.
In spindle motors disclosed in Unexamined Patent Publications H11-312359 and H06-178490, the yoke has a tubular yoke main body, and a fixing part extending from the end of the yoke main body radially inward. The yoke of this type has, as shown in FIG. 12, a fixing part 533b is in contact with a rotor magnet 534 in the axial direction, so that a fringe magnetic field 95 (dashed lines in the figure) of a main pole at a radially inner portion of the rotor magnet 534 is attracted to the edge of a radially inner portion of the fixing part 533b of the yoke 533. Consequently, the flux linkages 94 (bold lines in the figure) to a stator core 505 is decreased and the magnetic flux is likely to be leaked toward the discs 81, so that magnetic noises are applied to a head 90 mounted on an actuator 91. As a result, it is difficult to perform normal recording and reproduction in a magnetic recording and reproducing apparatus, such as hard disc drives.
Furthermore, since the top face of the yoke and the rotor magnet are closely located, the amount of leakage flux from the magnet to the yoke is likely to fluctuate depending on the form accuracy of the yoke or the rotor magnet end face. Consequently, the amount of flux linkages with the stator core is likely to vary so that motor vibrations are generated.
Furthermore, in a case that the upper end of the rotor magnet is in contact with the fixing part of the yoke, if the thickness of the fixing part of the yoke is varied and the height of the contact surface of the fixing part is varied, positions of the rotor magnet in the axial direction are not constant along the circumferential direction. As a result, the relative height between the rotor magnet and the stator core varies, so that axial vibrations or cogging vibrations are generated between the stator core and the rotor magnet due to the magnetic imbalance in the axial direction.
Typically, when the hydrodynamic bearings are used for the spindle motors, in order to reduce the variation of the position of the rotor in the axial direction, an attraction ring of magnetic material is located on the base plate opposite the rotor magnet in the axial direction. However, if the position of the rotor magnet in the axial direction is not constant along the circumferential direction, the attractive force of the attraction ring is varied, thereby varying the bearing performance. In addition, if the position of the rotor magnet in the axial direction is not constant along the circumferential direction, an inclined force is applied to the rotor. When the inclined force is changed according to the rotational phase, rotor vibrations are generated.
Consequently, in the conventional spindle motors, it is very important to reduce vibrations during the rotation by reducing the leakage flux from the yoke or stabilize the position of the rotor magnet in the axial direction.
In contrast, the yoke is cylindrical in the spindle motor disclosed in Unexamined Patent Publication 2004-263863, and the upper end of the rotor magnet is in contact with a projection of the nonmagnetic hub lower end. In this case, the degree of fringe magnetic field of the main pole at the radially inner portion of the rotor magnet shown in FIG. 12 attracted toward the yoke is decreased, and the degree of variation of the flux linkages to the stator core is decreased. However, the structure shown in the document has the following problems.
1) Since a motor structure of what is called in-hub type is employed, it is necessary to make the yoke thick to reduce the leakage flux to the disc positioned around the yoke. As a result, the outer diameter of the rotor magnet becomes smaller, so that electromagnetic conversion efficiency of the motor is extremely deteriorated and the motor current is increased.
2) Since a structure is employed in which a “groove-shaped” recess in cross section is formed at the deep bottom of a cup shape structure and the inner cylindrical surface of the yoke is bonded to the recess, it is necessary to use a form tool. In this case, as shown in FIG. 13(a), since nose R of a form tool 96 is shape-transferred to the inner circumference of a recess 535b of a hub 535, as shown in FIG. 13(b), it is necessary to form a large C-chamfering 533c on the upper surface of the radially inner portion of the yoke 533 to reliably fit the upper end of the yoke 533 into the recess 535b. Accordingly, the adhesion length between the yoke 533 and the recess 535b of the hub 535 is not enough, so that if a high impact is applied during the motor driving, the yoke 533 might come off. In addition, the form tool 96 needs to be repolished frequently if the number of products to be processed increases, thereby increasing the working cost.
3) In order to avoid the above-described problem, as shown in FIG. 14(a) and (b), it is necessary to prepare two kinds of cutting tools 97a and 97b to process the inner circumference 535c and the outer circumference 535d of the recess 535b, respectively, the tips of them being slender. Accordingly, at the process it is likely that chattering vibrations are generated in the cutting tools so that the machining accuracy is deteriorated or the surface roughness is developed, or burrs remain. As a result, it is expected that the concentricity will be deteriorated or the back yoke adhesion margin is deviated when the back yoke is bonded. If the remained burrs come off during the transportation or the operation, they might go between the disc and the head to damage the disc or destroy the device.
Furthermore, when the back yoke is fixed to the hub, it is necessary to use bonding or press-fit. The bonding entails problems such as the preparation of the clean oven and a large increase in the power during the production. The press-fitting entails problems such as the above-described machining accuracy and the burrs so that it is difficult to normally perform the press-fitting.
In conclusion, in the conventional spindle motors, it is very important to improve the electromagnetic conversion efficiency of motors, make it easy to process a yoke mounted on the hub, or reliably position the yoke and decrease the cost of manufacturing.